316 University of California Publications in Geology [Vol. 13 



The two posterior premolars in the lower dentition are apparently 

 less closely spaced than in Didelphys. Each of the premolars possesses 

 a very prominent cusp behind which is situated a distinct ledge. 



Fig. 5. Perathcrium merriami, n. sp. Inferior cheek-tooth series, no. 24240, 

 Mus. Palae. Coll., lateral and occlusal views, X 4. John Day beds, Logan Butte, 

 Oregon. 



The lower molar series is similar to that in Marmosa. In each 

 molar the trigonid and talonid are distinct. The protoconid is a prom- 

 inent cusp. A small eingulum is present along the antero-external 

 border of the trigonid, comparable to that seen in molars of some 

 species of Marmosa, and is not so well developed as in Didelphys. 

 The eingulum is apparently but faintly indicated on molar teeth of 

 specimen 5259, Amer. Mus. Nat. Hist. Coll., referred to Peratherium 

 fugax. In M 1 of P. merriami the antero-external eingulum barely 

 reaches the protoconid. In the two following molar teeth, however, 

 the ledge extends farther to the outer side, but does not quite reach 

 the middle of the external surface of the protoconid. Unfortunately 

 the trigonid in M 5 has been destroyed. The talonid in this tooth is 

 compressed transversely and consists of three distinct cusps. The 

 hypoconulid is less prominent than either the hypoconid or the ento- 

 conid. 



Measurements of Dentition 



Length of upper tooth row, P- to M 1 inclusive 

 Length of lower tooth row, to Mj inclusive 



al0.4 mm. 



1L> 



